1. Field of the Invention
The present invention relates to a photoelectric converting device, more particularly a photoelectric converting device obtained by forming an electrode across an insulating layer on a control electrode area of a semiconductor transistor, and a method for producing the same.
2. Related Background Art
In conventional semiconductor image pickup devices, the cell size of the sensor portion, constituting a separate device, cannot be considered sufficiently small in comparison with the advanced degree of integration of a driving circuit portion. For example, in contrast to the usual cell size of about 70 .mu.m.sup.2 in a 256K DRAM, that of a semiconductor image pickup device for example of 500.times.500 pixels is as several times as large as 200 to 300 .mu.m.sup.2. An increase in the cell size increases the chip area and therefore reduces the number of chips per wafer. Also in comparison with usual digital logic circuit devices, semiconductor image pickup devices are associated with much lower production yields because they have to satisfy optical and analog requirements. For these reasons the cost per chip of the sensor portion is inevitably higher than that of DRAMS or similar devices.
The reduction of cell size can be achieved either by a scale-down method, in which dimensions are proportionally reduced, or by a cell improvement method, in which the number of cells constituting components is reduced.
The cell size has been reduced year after year with the progress of microintegration technology, but the rate of size reduction is averaging in the range 0.88 times per year. Thus a drastic size reduction cannot be expected in the former method. More detailedly, though a size reduction results in a reduction in the chip area and thus an increase in the number of chips per wafer, the production yield is lowered due to increased difficulty of process control. Consequently, the number of defectless chips per wafer will show a peak at a certain cell dimension.
On the other hand, the latter method can achieve a drastic reduction in size. For example, in case of a random access memory, a memory unit originally composed of 6 to 8 transistors is now simplified as a transistor and a capacitor.
The latter cell improvement method is also effective in the field of semiconductor image pickup devices. For example charge-coupled image pickup devices (CCD) are now superior to MOS image pickup devices in simplicity and integration, by isolating each carrier accumulating portion not by a mechanical structure but by an electrically formed potential well. Also the charge transfer portion does not require sources and drains in the MOS structure and can therefore achieve a higher degree of integration. Though CCD's can achieve a higher level of integration in comparison with MOS devices, they are disadvantageous in production yield, because, in CCD's, a defect in the charge transfer portion affects the image information of an entire corresponding line. Also defects in the substrate cannot be tolerated because the information transfer is conducted not by metal wirings but by the interior of the substrate itself, and these facts deteriorate the production yield and raise the production cost. Under such a situation, CCD's and MOS devices are now competing with each other in the field of semiconductor image pickup devices.